The present invention relates to calenders in general, and more particularly to improvements in means for mounting and moving the uppermost roll of a stack of superjacent rolls in a machine for calendering webs of paper, textile material or the like.
It is already known to mount the bearings for the end portions of the lowermost calender roll on hydraulic motors which maintain such bearings in abutment with the undersides of suitable stops on the frame or support of the calender. It is further known to employ upright feed screws which can move the bearings for the end portions of the uppermost roll downwardly so that the uppermost roll can bear against the roll therebelow with a pressure which is required for proper treatment of the running web. For example, U.S. Pat. No. 2,861,504 granted Nov. 25, 1958 to Kane discloses a calender wherein the lower end portions of the feed screws are connected with the bearings for the uppermost roll. The feed screws serve to adjust the position of the uppermost roll in order to compensate for reductions of roll diameters as a result of wear upon and surface treatment of such rolls. Furthermore, the feed screws select the pressure with which the neighboring rolls bear against each other so that the web which is advanced through the nips of the rolls is subjected to requisite smoothing, satinizing or other treatment. It has been found that such construction does not allow for accurate adjustment of the uppermost roll with reference to the support and/or with reference to the roll or rolls therebelow.
U.S. Pat. No. 3,364,848 granted Jan. 23, 1968 to Muller discloses a paper satinizing calender wherein the bearings for the end portions of the uppermost roll are biased downwardly by the piston rods of double-acting hydraulic cylinder and piston units. The cylinders are mounted on the support at a level above the uppermost roll. The piston rods perform the functions of the aforementioned feed screws, namely, to compensate for a reduction of the diameter of each surface-treated roll as well as to apply the necessary pressure to the web which travels through the nips of neighboring rolls. A drawback of the machine which is disclosed in the patent to Muller is that the strokes of the piston rods are quite substantial, often in the range of up to 800 mm if the calender comprises a substantial number of rolls. Consequently, if the lower end portions of extended piston rods are subjected to pronounced transverse (substantially horizontal) stresses which tend to move the piston rods out of axial alignment with the respective cylinders, the pistons at the upper ends of the piston rods are likely to jam in the cylinders and the piston rods are likely to apply substantial forces to their guide means which are installed at the lower ends of the associated cylinders. The aforementioned transverse stresses can develop for a variety of reasons, for example, due to tolerances in the machining of guide means for the bearings of the uppermost roll and/or due to unavoidable vibrations which develop when the calender is in use. Such stressing of the lower end portions of the piston rods entails pronounced wear upon the cylinders, upon the piston, upon the surfaces of the piston rods and upon the sealing means therefor as well as undesirable instability of the respective bearings. As the wear progresses, the rate at which the hydraulic fluid escapes from the cylinders increases and the leaking fluid is likely to contaminate parts of the machine as well as the running web. In order to eliminate or repair the resulting damage, the calender must be arrested at frequent intervals and for extended periods of time with attendant pronounced losses in output. Attempts to reduce leakage by resorting to complex and numerous sealing elements have met with minimal success. In fact, the provision of more elaborate and/or additional seals entails an increase in frictional forces which prevent accurate adjustment of the forces with which the rolls bear against the running web when the calender is in use. Furthermore, the just mentioned frictional forces are not identical at both ends of the uppermost roll and/or their magnitude varies with changes in the level of the uppermost roll so that the machine does not allow for predictable and reproducible selection of forces which are applied to the treated material.